1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly, the present invention relates to vertical channel transistors and semiconductor memory devices which include vertical channel transistors.
2. Description of the Related Art
FIG. 1 is a schematic cross-sectional view of the PMOS and NMOS planar channel transistors of a conventional CMOS device. As illustrated, the NMOS planar channel transistor is generally defined by n+ type source/drain regions 101 and 103 formed in the surface of a p-type substrate 100, and an n+ type poly-silicon gate electrode 104 is located over an p-channel region 102 of the NMOS planar channel transistor. The PMOS planar channel transistor is generally defined by p+ type source/drain regions 111 and 113 formed in the surface of an n-type well 100′ of p-type substrate 100. An n+ type poly-silicon gate electrode 114 is located over a n-channel region 112 of the PMOS planar channel transistor.
FIG. 2 is a graph generally illustrating the relationship between threshold voltage Vth and channel region impurity concentration of NMOS and PMOS planar channel transistors. As is well-understood in the art, the threshold voltages (Vth) of the NMOS and PMOS planar channel transistors can be engineered by precisely controlling impurity concentrations in the channel regions 202 and 212 (FIG. 1), respectively. In a general CMOS operation, the threshold voltage Vth of the NMOS transistor is positive, while the threshold voltage Vth of the PMOS transistor is negative. Accordingly, to realize a CMOS device operation, a precise channel implantation process is generally necessary in the NMOS planar transistor to bring the threshold voltage Vth from negative to positive (see FIG. 2).
The PMOS and NMOS transistors of FIG. 1 are referred to here as “planar channel” transistors since the channel regions 202 and 212 extend along the planar (or horizontal) surface region of the substrate 101. More recently, however, in an effort to increase device integration, “vertical channel” transistors have developed in which the channel regions thereof extend vertically relative to the horizontal substrate surface.
FIG. 3A is a schematic cross-sectional view of a conventional device having NMOS and PMOS vertical channel transistors, and FIG. 3B is perspective view of the same. In FIGS. 3A and 3B, like elements are identified by like reference numbers.
Referring to FIGS. 3A and 3B, the NMOS vertical channel transistor includes a p-type vertical channel layer 302 formed on a p-type substrate 300, a first n+ type source/drain layer 301 formed in the surface of the p-type substrate 300 and surrounding the p-type vertical channel layer 302, a second n+ type source/drain layer 303 formed over the p-type vertical channel layer 302. The NMOS vertical channel transistor further includes an n+ type poly-silicon gate electrode 304 which surrounds the p-type vertical channel layer 302. Although not shown, a gate dielectric is interposed between the n+ type poly-silicon gate electrode 304 and the p-type vertical channel layer 302.
The PMOS vertical channel transistor includes a n-type vertical channel layer 312 formed on a n-well 300′ into the p-type substrate 300, a first p+ type source/drain layer 311 formed in the surface of the n-well 300′ and surrounding the n-type vertical channel layer 312, a second p+ type source/drain layer 313 formed over the n-type vertical channel layer 312. The PMOS vertical channel transistor further includes an n+ type poly-silicon gate electrode 314 which surrounds the n-type vertical channel layer 312. Also, a gate dielectric (not shown) is interposed between the n+ type poly-silicon gate electrode 314 and the n-type vertical channel layer 312.
Typically, the vertical channels 302 and 312 are defined by pillar-like structures having generally circular horizontal cross-sections, and the poly-silicon gate electrodes 304 and 314 are cylindrical and completely surround the respective vertical channels 302 and 312. Also, the first source/drain layers 301 and 311 and the second type source/drain layers 303 and 313 are typically defined by plate-like structures also having generally circular horizontal cross-sections. In the example of FIGS. 3A and 3B, outer diameters of the poly-silicon gate electrodes 304 and 314 substantially conform to outer diameters of the first source/drain layers 301 and 311, and outer diameters of the vertical channels 302 and 312 substantially conform to outer diameters of the second type source/drain layers 303 and 313.
One drawback of vertical channel transistors is that it is difficult to accurately and reliably implant impurities into the pillar-like structures of the p-type and n-type vertical channel layers 302 and 312. This is especially problematic with respect to the NMOS vertical channel transistor. That is, as explained previously in connection with FIG. 2, it is generally necessary to execute a channel implantation process in order to establish a positive threshold voltage Vth for an NMOS device. However, any attempt to implant ions in the p-type vertical channel 302 will likely result in a non-uniform ion density distribution, which can cause intended variations in the threshold voltage Vth. This problem is amplified as the pillar diameter is of the p-type vertical channel 302 is decreased to enhance device integration.
Accordingly, conventional NMOS vertical channel transistors generally operate at a negative threshold voltage Vth (e.g., −0.4V). It is therefore necessary to configure the corresponding device with a special control block capable of generating a negative voltage to turn off the NMOS vertical channel transistors. In addition, it is difficult to realize a CMOS operational mode using conventional NMOS vertical channel transistors, since a CMOS operational mode generally requires a positive threshold for the NMOS transistors.